An α-olefin low polymer is a useful substance that is widely used as a monomer raw material of olefin-based polymers, as a comonomer of various polymers, and as a raw material of plasticizers, surfactants, lubricating oils, and so on. In particular, 1-hexene obtained by low polymerization reaction of ethylene is useful as a raw material linear low-density polyethylene.
The α-olefin low polymer is typically produced by a method of subjecting an α-olefin to low polymerization reaction in the presence of a catalyst and a solvent.
For example, Patent Document 1 describes a method of producing an α-olefin low polymer composed mainly of 1-hexene in a high yield and a high selectivity by using a chromium-based catalyst and an n-heptane solvent.
Since the low polymerization reaction for obtaining an α-olefin low polymer composed mainly of 1-hexene is an exothermic reaction, there have hitherto been studied industrial methods for continuously producing an α-olefin low polymer while removing reaction heat generated within a reactor.
Patent Document 2 describes a method of producing an α-olefin oligomer having an average molecular weight of 50 to 350 by oligomerization of the ethylene in the presence of a catalyst, in which a gas of a gas phase within a reactor is used as a coolant, a part of the gas of the gas phase within the reactor is cooled by a condenser not coming into direct contact with a liquid phase, and the polymerization heat is removed by a condensed liquid.
In addition, Patent Document 3 describes a method of producing an α-olefin low polymer, in which a gas within a reactor is introduced into a heat exchanger, a condensed liquid obtained from an outlet of the heat exchanger and the gas are circulated into the reactor and describes that in order to suppress the entrainment of a reaction liquid in a gas phase part, a linear gas velocity of the gas phase part within the reactor is controlled to a predetermined range.
The shell and tube type heat exchanger that is used in the present invention has a structure in which a large number of pipes (tubes) are disposed in a hermetically sealed outer cylinder (shell) and is also called a tubular condenser, and in view of the fact that a wide heat transfer area can be ensured in a small capacity, it is also industrially used as a cooling condenser of gas. For example, Patent Document 4 describes a shell and tube type heat exchanger applied for a recovering apparatus of a polymerizable monomer vapor.
FIG. 11 is a schematic vertical cross-sectional view of a conventional vertical shell and tube type heat exchanger 90.
This heat exchanger 90 includes a cylindrical shell (trunk part) 91 whose cylinder axial direction is set in the vertical direction; an upper tube plate 92 and a lower tube plate 93 disposed on the upper side and the lower side of the shell 91, respectively; a large number of tubes 94 installed between the tube plates 92 and 93; a top cover 95 disposed on the upper side of the upper tube plate 92; and a bottom cover 96 disposed on the lower side of the lower tube plate 93. A gas supply nozzle 95a of a gas to be condensed is provided in the top of the top cover 95, and a takeout port 96a of a condensed liquid is provided in the bottom of the bottom cover 96.
In addition, a discharge port 96b of a non-condensable gas is provided on the side face of the bottom cover 96, and a cover 96c for preventing outflow of a condensed liquid is provided inside of the bottom cover 96 of an opening of this discharge port 96b. 
In addition, an inflow port 91a of cooling water is provided in a lower portion of the side face of the shell 91, and an outflow port 91b of cooling water is provided in an upper side thereof.
The peripheries of the tube plates 92 and 93 are sandwiched between flanges (symbols omitted) provided on the lower end of the top cover 95, the both upper and lower ends of the shell 91, and the upper end of the bottom cover 96 and fixed by bolts (illustration omitted) penetrating therethrough.
The interior of each of the tubes 94 not only communicates with the interior of a reception chamber 97 surrounded by the upper tube plate 92 and the top cover 95 but also communicates with the interior of a takeout chamber 98 surrounded by the lower tube plate 93 and the bottom cover 96. The upper end and lower end of each of the tubes 94 are fixed to the tube plates 92 and 93, respectively by means of welding or the like.
The gas to be condensed flows into in the reception chamber 97 from the gas supply nozzle 95a and passes through the interior of each of the tubes 94. During a time when the gas to be condensed passes through the interior of the tube 94, it is cooled by cooling water and condensed, and the condensed liquid goes through the takeout chamber 98 and is then withdrawn from the takeout port 96a. The non-condensable gas is discharged from the discharge port 96b. 
There is also a case where the reception chamber 97 of the gas to be condensed is provided with an impact plate (baffle plate) in order to disperse the gas to be condensed having flown thereinto against the side of the upper tube plate 92.